In the gnarled wreckage of the World Trade Center and the quiet of his laboratory, Professor Abolhassan Astaneh-Asl hopes to learn how to guard steel skyscrapers against bombs, earthquakes and airplane crashes.

An expert on steel buildings, the University of California at Berkeley civil and environmental engineering professor recently edged his way through the jungle of tangled steel, shattered marble and pulverized concrete that was once New York City's boldest emblem.

He scanned the ruin for clues to a mystery: Why and how did the towers collapse on Sept. 11 after terrorists rammed commercial airliners into them?

And based on that information, can skyscrapers be made less vulnerable?

He thinks they can, perhaps using a new type of building barrier -- a concrete-and-steel "shear wall" -- that he is testing in his lab at Berkeley.

In the 19th century, steel-making technology inaugurated the age of skyscrapers. Steelmakers prophesied that humanity would begin building skyward -- literally into the clouds -- thanks to the super-strong metal.

Not strong enough, though, as the events of Sept. 11 showed.

Astaneh displayed a banana-shaped, rust-colored piece of steel. Somehow, it twisted like toffee during the terrorist attack.

"I keep it on my desk to show students what steel can do," he said.

Until recent years, engineers tended to assume that steel buildings were relatively safe against earthquakes and small bombs.

Doubts emerged, though, partly because steel structures proved surprisingly weak during California's Northridge quake of 1994. The big shock came when terrorists proved it doesn't take a low-yield nuclear weapon to topple a big office building.

When terrorists rammed two airplanes into the towers, burning jet fuel gushed into the structures. The blaze heated and weakened their steel beams. Soon both towers collapsed like souffles.

The twin towers were exceptionally well designed and built, Astaneh said, calling the trade center "the best-designed building I have ever seen." He was especially struck by the strength of a building column that an airplane engine crashed into. Although the engine "took a bite" out of the column, it remained standing -- until heat from the jet fuel weakened it.

Astaneh said the towers might have withstood the attack had they been reinforced properly with 6-inch-thick concrete-and-steel shear walls. The shear walls could be installed in both existing and new buildings in order to strengthen their regular exterior skin.

Such reinforcement would cover the giant expanses of clear glass that are the hallmark of many modern buildings. But the materials could be designed to allow smaller windows.

The idea is to create a tough outer membrane sufficient to withstand the shock of a direct impact and minimize the amount of jet fuel that squirts into a building.

Astaneh is testing such slabs in his cavernous lab inside Davis Hall on the Berkeley campus.

A loud "bang" echoed through the room as a steel bolt snapped on the experimental apparatus -- the result of 600,000 pounds of pressure exerted by a giant piston.

By Astaneh's estimate, an effective way may be found to reinforce key buildings without boosting their construction costs by more than 1 to 2 percent.

Given the likelihood of more terrorist attacks, it might be worth the extra cost, he said, showing reporters a bolt that he recovered from the ruins in New York.